The GFX and Physics engines are both 4.5/5 stars in terms of performance and features, really awesome! I miss grabber cranes tho.
What's really killing this game rn is the engine that handles the game mechanics, such as inventory, quest state and such. I mean you need to go all the way back to the design stage and go over everything with a fine tooth comb and make sure that the logic is correct and that it can recover from inconsistent save states and that it uses proper transaction guarding for things that might crash or be interrupted such as spawning a vehicle, moving between maps, or any activity in MP.
Just patching things is not a viable approach with the current state of the game. You need to review things from a higher level and make sure that everything is being done the right way.